ENH: Add api for covstruct. Clear __init__. Closes #1917.
ENH: Add api to genmod. Fix cov_struct import.
REF: Explicit imports
REF: Move cov_struct to simpler import.
Merge pull request #1981 from jseabold/cov_struct-import

ENH: Add api for covstruct. Clear __init__. Closes #1917.